Driveline components, such as gears, for example, are traditionally formed from a low carbon content steel. One example of a gear material is SAE 8822H, which is a carburizing grade alloy steel. SAE 8822H has the following chemical composition, in combination, by weight: 0.19-0.25% carbon (C), 0.70-1.05% manganese (Mn), 0.15-0.35% silicon (Si), 0.35-0.75% nickel (Ni), 0.35-0.65% chromium (Cr), 0.30-0.40% molybdenum (Mo), no more than 0.035% phosphorous (P), and no more than 0.040% sulfur (S), with the balance being essentially iron (Fe).
Gear steels, such as SAE 8822H, are specially designed carburization grade steels that are alloyed-low carbon content steels (0.10-0.27% carbon), which traditionally are expensive. Carburizing is a process in which carbon is added to a surface of an iron-base alloy by absorption through heating the alloy at a temperature below a melting point of the alloy, while providing contact with carbonaceous solids, liquids, or gases. In order to achieve desired final hardness and surface characteristics, the SAE 8822H material is carburized, quenched, and tempered.
An example of one current process for carburizing a traditional gear steel, such as SAE 8822H, is as follows. The gear steel is subjected to carburization for 22 hours at 1750° F. for a pinion gear and for 14 hours at 1750° F. for a ring gear. The atmosphere has approximately 1% carbon potential. After carburization, the gear steel is quenched in an oil bath. Additional processing steps, such as tempering and/or shot peening, for example, are then performed to achieve desired final material characteristics.
The process provides a relatively uniform case depth for the ring and pinion gears, which results in requiring only a pitch line case depth to be defined. Case depth at a gear tooth root is not typically defined. Core hardness for ring and pinion gears made by this process is typically no more than 45 Rockwell C at the pitch line, and surface hardness is approximately 58-63 Rockwell C. Microstructure 0.010 inches beneath the surface is martensite and retained austenite. Residual compressive stress is typically less than 50 ksi at the gear tooth root and less than 100 ksi after shot peening.
Thus, carburization for gears and other driveline components is a prolonged process and can take as long as ten to twenty-four hours, depending on case depth requirements. Prolonged processing and expensive steel grades increase manufacturing costs for gears and other driveline components.
Also, the prolonged carburization process causes non-martensite transformation products (NMTP) and intergranular oxides (IGO) to form at a surface of the component. NMTP and IGO adversely affect bending fatigue strength and wear resistance. NMTP is also referred to as surface high temperature transformation product (HTTP). The appearance of HTTP/NMTP results in a softer material at a surface of the component, which is detrimental to wear resistance. IGO is very brittle and is more susceptible to micro-cracking. Thus, IGO results in lower compressive residual stress, which reduces bending fatigue and wear resistance. The occurrence of both NMTP and IGO can significantly reduce service life of the component.
An example of a process used to achieve desired material characteristics for a high carbon content steel (0.60-0.80% carbon) is thru-surface hardening (TSH). This process heats the steel in a controlled furnace atmosphere for about 40 minutes to one hour, and then subsequently quenches the steel in a water based solution. This process provides an irregular case profile and has a root case depth of approximately 0.045 to 0.060 inches for gears. The gear pitch line core hardness is 55 Rockwell C and surface hardness is 58-63 Rockwell C. Microstructure 0.010 inches beneath the surface is martensite only for 0.60% carbon steel, and is martensite and retained austenite for 0.80% carbon steel. This process is undesirable because the core hardness of 55 Rockwell C makes machining very difficult. Further, when the microstructure consists mostly of martensite at the surface, wear resistance is adversely affected.
It is desirable to have an improved process for a driveline component material that does not require prolonged carburization or thru-surface hardening, is less expensive, and provides improved surface characteristics for the driveline components, as well as overcoming the other above-mentioned deficiencies in the prior art.